When a well such as an oil or gas well has been drilled, it is often desired to isolate the various producing zones form each other or from the well itself in order to stabilise the well or prevent fluid communication between the zones or shut off unwanted fluid production such as water. This isolation is typically achieved by installing a tubular casing in the well and filling the annulus between the outside of the casing and the wall of the well (the formation) with cement. The cement is usually placed in the annulus by pumping a slurry of the cement down the casing such that it exits at the bottom of the well and passes back up the outside of the casing to fill the annulus. While it is possible to mix the cement as a batch prior to pumping into the well, it has become desirable to effect continuous mixing of the cement slurry at the surface just prior to pumping into the well. This has been found to provide better control of cement properties and more efficient use of materials.
The cement slurries used in such operations comprise a mixture of dry and liquid materials. The liquid phase is typically water and so is readily available and cheap. The solid materials define the slurry and cement properties when added to the water and mixed, the amount of solid materials in the slurry being important. Since the liquid phase is constant, the amount of solid material added is usually monitored by measuring the density of the slurry and maintaining this at the desired level by controlling the amount of the solid material being added. FIG. 1 shows a schematic diagram of a prior art mixing system. In the system of FIG. 1, mix water is pumped from a feed supply 10 via a pump 12 to a mixer 14 which feeds into a mixing tub 16. The feed supply 10 comprises a pair of displacement tanks 11, 11′ each with separate outlets connected to a valve 13 which in turn feeds the pump 12. Two methods are commonly used to determine the amount of water supplied:    1. Proximity switches installed on the shaft of the pump 12 count a number of pulses per rotation. Each pulse corresponds to a displacement volume. This method is sensitive to pump efficiency.    2. Displacement volume is measured by counting the number of tanks pumped down-hole. This measurement method is sensitive to human error in level reading, switching from on tank to another and tank exact capacity. Even more an error in the number of tanks counted can have many consequences (over displacement can result in wet shoe, under displacement can result in no pressure bump or cement left in the casing).
Solid materials are delivered to the mixer 14 from a surge can 18 or directly from a cement silo via a flow control valve 20 and are carried into the mixing tub 16 with the mix water. The contents of the mixing tub 16 are recirculated through a recirculation pipe 22 and pump 24 to the mixer 14. The recirculation pipe 22 also includes a densitometer 26 which provides a measurement of the density of the slurry in the mixing tub 16. An output 28 is provided for slurry to be fed from the mixing tub 16 to further pumps (not shown) for pumping into the well. Control of the slurry mixture is achieved by controlling the density in the mixing tub 16 as provided by the densitometer 26 by addition of solid material so stay at a predetermined level for the slurry desired to be pumped. The densitometer 26 is typically a non-radioactive device such as a Coriolis meter.
While this system is effective for slurries using materials of much higher density than water, it is not effective for slurries using low density solid materials, especially when the density of the solids is close to that of water. In such cases, a density measurement is not sensitive enough to control the amounts of solid material added to the necessary accuracy.
The present invention seeks to provide a mixing system which avoid the problem of density measurement described above.
In its broadest aspect, the present invention comprises using a measurement of the solid fraction of a fluid as it is being mixed to determine the ratio of the solid and liquid components added to the slurry.
The invention is particularly applicable to the mixing of borehole cement slurries, in which case, solids fraction is determined as (slurry vol−water vol)/slurry vol. An alternative but related parameter is porosity, determined as water vol/slurry vol (porosity+solids fraction=1).
A system for mixing cement in accordance with the invention comprises a liquid (water) supply including a device for measuring the amount of liquid supplied; a solid material supply; a mixer which receives the liquid and solid materials and includes an output for delivering materials from the mixer to a delivery system; a device for measuring the amount of material in the mixer; and a flow meter in the output; wherein measurements from the flow meters and the device for measuring the amount of material in the mixer are used to control the amount of solid material added to the mixer.
The flow meters can be mass flow meters or volumetric flow meters. Any suitable form of meter can be used, for example Coriolis meters or electromagnetic meters.
The mixer will typically include a tank or tub, in which case the device for measuring the amount of material in the mixer can be a level sensor. Such a level sensor is preferably a time domain reflectometry- or radar-type device although acoustic or float devices can also be used. It is preferred to mount such a device in an arrangement for damping transient fluctuations in the tank level, for example in an arrangement of concentric slotted tubes. An alternative or additional form of sensor can be a load cell which can be used to indicate the weight of the tank, or a pressure sensor.
The device for measuring the amount of liquid supplied can be a flow meter or a level sensor of the types described above. When the liquid supply includes one or more displacement tanks, a level sensor is preferred
Where the mixer includes some form of recirculation of the slurry through the tank, it is important that the output flow meter is downstream of this recirculation.
Where the solid materials comprise cement and other solid additives added separately to the mixer, separate flow meters can also be provided for each separate supply of additives.
In its simplest form, the measurement of solid fraction is used as a guide for the operator to add solids, particularly cement, to the slurry as it is mixed. In more advanced versions, the calculation of solids fraction is used to control the addition of solids directly by means of an automatic control system.
The invention also provides an improved method for calculating displacement volume from a system comprising at least one displacement tank, comprising measuring the level of liquid in the tanks over time and calculating displacement as:Σ(V(h1n)−V(h2n))where:                V(h) is the exact volume of the tank at level (h);        h1n is the start level of the n th displaced tank volume; and        h2n is the stop level of the n th displaced tank volume.        